VAC comes with audio repeaters that are separate programs. You need to search these up on your computer. They install alongside VAC automatically.
Search "MME" on your PC. You should see something like "MME Audio Repeater". Open this program three times.
In the first window of MME you opened, for the top dropdown select your Speakers. For the next dropdown, select Line 1.
In the second window of MME you opened, for the top dropdown select Line 1. For the next dropdown, select Line 2.
In the third window of MME, for the top dropdown select your microphone input. For the next dropdown, select Line 1.
After all the dropdowns have been filled and selected, click "Start" on all three audio repeaters.
Now, your speakers and microphone outputs will go straight into Line 2. So, you are essentially combining the two.
Note: If you cannot find Line 1 and/or 2, go to your Audio Mixer and make sure these are enabled and that Line 2 is set as your default device under "Recording" so you may use it in games and override your actual microphone. If you are using foobar2000 to play music through the microphone, make sure you select the output as Line 2.
Let me know if you have any questions. I typed this on my phone and did it from memory, so some information may be off.